A one-beam push-pull method is known as a method of making laser light from an optical head device follow an information track on an optical disc. In this method, reflection light of the laser light which has been diffracted at an information track on the optical disc is detected by two-divided light receiving surfaces of a photodetector, and an objective lens is shifted in a radial direction of the optical disc so that a tracking error signal which is a difference between detection signals approaches zero.
However, in the conventional one-beam push-pull method, if the objective lens shifts in the radial direction of the optical disc, a positional relationship between the objective lens and the photodetector relatively changes. Consequently, a light spot applied on the photodetector shifts and an offset occurs in the tracking error signal.
An art of canceling an offset is proposed in Patent Document 1, for example. According to the art described in the Patent Document 1, positive/negative first-order diffraction light from a polarizing hologram is separated, each of the separated positive/negative first-order diffraction light is detected by a photodetector which has a light receiving surface larger than a shift amount of an irradiation position, and a tracking error signal without offset is obtained by using a detection signal resulting from the detection.